Apparatus for and method of installing subsea components

ABSTRACT

An apparatus for installing subsea components includes the use of a motion compensator to counterbalance and offset load on the component and lowering wire caused by movement of the vessel as a result of waves and wind.

BACKGROUND OF THE INVENTION

[0001] To install heavy subsea components into the seafloor, a floating installation vessel is typically employed. Since the components lowered vary in weight from several tons to 200 tons or more, installation vessels such as a barge commonly utilize a crane or a derreck. Other vessels have a semi-submersible hull where the component may be lowered through an aperture in the hull (“moonpool”) into the water and down to the seabed. To perform such operation with this vessel, a crane or derreck is used depending on the component. For example, a crane will lower a wet block or steel wire while the derreck lowers the drill string.

[0002] More recently, Anchor Handling Tug and Supply (“AHTS”) vessels are used to install heavy subsea equipment. These vessels have a relatively small ship-like hull, and often do not use a crane but rather use winches to lower components into the sea. The AHTS vessel has less capacity in load, storage and accommodation than a typical barge which are also used for offshore applications.

[0003] While derreck and crane barges are frequently used all around the world to install heavy components, use of these vessels is typically much more expensive than the AHTS-type vessels or work boats. Indeed, in recent years the cost of using a derreck and crane barges may be as much as 3 to 10 times more expensive per day in comparison to the relatively smaller AHTS vessel. Hence, the major drawback to using larger vessels to install the same components is cost. Typical installation vessel particulars are provided in the table below. Typical Typical AHTS Derreck Typical Crane Typical Crane Vessel Barge Barge (a) Barge (b) Hull Type: Monohull Semi- Monohull Semi- submersible submersible Length: 279 ft 312 ft 497 ft 649 ft Beam:  65 ft 191 ft 151 ft 285 ft Depth:  26 ft  95 ft  41 ft 143 ft Operating  22 ft  50 ft  31 ft  90 ft Draft: RELATIVE  1  3  6  10 COST FACTOR:

[0004] Notwithstanding, while the AHTS vessel is more cost efficient, there are problems associated with lowering subsea components from the floating platform of this vessel that crane and/or derreck barges do not suffer.

[0005] For example, crane barges do not risk overboarding the component, or the failure of the component to clear the vessel structure. On a barge, the crane usually lowers the component into the water, clear of any vessel structure. On the other hand, both derricks and AHTS vessels require relatively complicated procedures in order to ensure the component clears the vessel.

[0006] Another problem is the relative motion between the component and the sea bed prior to touch down. Heave in particular can cause damage to the component and other structures around it, if it is moving up and down uncontrollably. Crane barges and vessels with derrecks typically have one or more heave compensators built into the crane or derreck which allows for installation during relatively rough weather. Similarly, the larger semi-submersible vessel has relatively good motion characteristics by design so they can typically work in harsher environments when compared to smaller and/or monohull vessels. On the other hand, smaller installation vessels do not use heave compensation at all which means they must rely on calm weather during installation operations.

[0007] A further problem is the twisting of the component and torque on the lowering wire prior to touch down. Most components require a specified heading. If the lowering line twists, the component will also twist and thus change headings. Excessive twisting may also cause damage to the lowering lines. The type of lowering line will determine the amount of twisting of the component. Steel wires typically have tension-induced torque characteristics, which can increase the chance of twisting. Crane barges typically lower components with a wet block, in which the lowering wire is reeved through the wet block in a way that prevents the component (and the block) from twisting up. Derreck vessels use wires that may twist. The derrick barge also uses drill string which is a combination of rigid pipes screwed together extending from the derreck down to the seafloor.

[0008] Notwithstanding the problems, as disclosed in U.S. patent application Ser. No. 09/627,873 assigned to Aker Marine Contractors, Inc., incorporated herein by reference, small vessels have been proven very useful in installing components such as subsea manifolds, suction piles anchors, suction pile foundations, plate anchors, templates, subsea trees, gravity base anchors and rigid jumpers. Examples of methods and apparatus used to install certain of these components are disclosed under various scenarios in U.S. patent application Ser. No. 09/627,873 and also in U.S. Pat. Nos. 6,009,825 incorporated herein by reference and U.S. Pat. No. 6,122,847 also incorporated herein by reference. Unfortunately, even in light of the novel and useful nature of these methods, certain problems still persist.

[0009] For example, for deep subsea installation, direction control remains a problem when installing components through the use of the method and apparatus described in U.S. Pat. No. 6,009,825. Torque on the component and lowering wire causes wire damage, alleviated in part only through the use of rotation resistant ropes or torque balanced wire. An example of torque balanced wire is multistrand or multilayer strand, difficult to handle, fragile and expensive. Indeed, care must be taken to avoid twisting into the rope during handling and installation.

[0010] Overboarding remains a problem when only winches are used as shown in U.S. Pat. No. 6,122,847. Moreover, control over the torque or twist of the component is missing unless a second vessel is used. Furthermore, as taught in U.S. patent application Ser. No. 09/627,873 while installation of components using a single vessel is preferred, the method described lacks the ability to control the torque on the component and is not suitable for deep sea installations particularly where the environment is harsh and/or the relative motion between the component and the sea floor maybe uncontrollable.

[0011] A need exists, therefore, for a cost effective way to install components into the sea bed without the current problems of overboarding the component, torque or directional control and the relative motion between the component and the sea bed.

BRIEF SUMMARY OF THE INVENTION

[0012] The present invention is an apparatus for and method of installing subsea components. The apparatus of the subject invention comprises a vessel having a motion compensator and an A-frame. The A-frame is movably connected to a spreader beam for lowering the component into the sea. The motion compensator controls varying loads on the component as a result of the movement of the vessel caused by wind and waves through a reciprocating wire, rope, or the like that is connected to the A-frame and spreader beam. The subsea component is connected to the spreader beam and lowered onto the seafloor.

[0013] The motion compensator acts to reduce the effect of heave on the component as it lowered to the seafloor and thus, reduces the potential for the component to bounce up and down on the floor. The spreader beam separates the lowering wire and assists in controlling component orientation. Use of the A-frame avoids overboarding the component as it is deployed into the water.

[0014] The subject invention is an improvement to the many known methods of installing subsea components as relative motion of the component is controlled by the motion compensator which counterbalances and offsets the variations in load on the component and lowering wire created by movement of the vessel and caused by wind and waves or heave.

BRIEF DESCRIPTION OF THE DRAWINGS

[0015]FIG. 1a is a side view of an embodiment of a vessel suitable for the subject invention.

[0016]FIG. 1b is a top view of an embodiment of the vessel suitable for the subject invention.

[0017]FIG. 2 is an isometric stern view of an embodiment of a vessel featuring the A-frame, motion compensator and spreader beam of the subject invention.

[0018]FIG. 3 is isometric top stern view of an embodiment of a vessel suitable for the subject invention.

[0019]FIG. 4 is isometric view of a deck of a vessel suitable for the subject invention.

[0020]FIG. 5a is a side of the A-frame in various positions.

[0021]FIG. 5b is a stem of an A-frame suitable for the subject invention.

[0022]FIG. 6 is a side view of a motion compensator suitable for the subject invention.

[0023]FIG. 7 is a top view of a motion compensator suitable for the subject invention.

[0024]FIG. 8a is a end view of a motion compensator suitable for the subject invention.

[0025]FIG. 8b is section b-b of FIG. 7.

[0026]FIG. 9 is an isometric view of a motion compensator suitable for the subject invention.

[0027]FIG. 10 is a flow schematic for a motion compensator suitable for the subject invention.

[0028]FIG. 11a is a top view of a vessel of the subject invention attached to component to be lowered in the sea.

[0029]FIG. 11b is a side view of a vessel of the subject invention attached to component to be lowered in the sea.

[0030]FIG. 12a is a top view of a vessel of the subject invention launching component to be lowered in the sea.

[0031]FIG. 12b is a side view of a vessel of the subject invention launching component to be lowered in the sea.

[0032]FIG. 13a is a stem view of a vessel of the subject invention lowering component in the sea.

[0033]FIG. 13b is a side view of a vessel of the subject invention lowering component in the sea.

[0034]FIG. 14 is a side view of an ROV embedding component, suction pile, into the seabed.

DETAILED DESCRIPTION OF THE INVENTION

[0035] The subject invention is a method and apparatus for launching, lowering and installing components on the sea floor comprising a single vessel 10, an A-frame 12, a spreader beam 14 and a motion compensator 16. The spreader beam 14 is movably connected to the A-frame 12 for lowering the component 22 into the sea. The A-frame 12 is connected to the motion compensator 16 via a wire 34 as shown in the figures, or rope, chain, cable, or the like. The motion compensator 16 counterbalances and offsets variations in load on the component 22 and wire 34 that are created by the movement of the vessel 10 and caused by wind and waves or heave.

[0036] The apparatus of the subject is useful in launching, lowering and installing a wide variety of components 22 otherwise referred to as equipment or structures. The components 22 include, but not limited to, manifolds, jumper spools, subsea trees, suction anchors, suction followers, and gravity base anchors.

[0037] For example, manifolds are typically employed at pipeline junctions and are relatively large structures containing pipes, valves and a support structure in various sizes and weights. Subsea trees are similar to manifolds in that they contain many pipes and valves and a supporting structure. Rigid jumpers are relatively short pipes (˜150 ft in length) pipes that carry fluid between two structures, such as between subsea trees and manifolds.

[0038] Suction anchors include suction caissons and suction piles. The term suction caisson is used to describe a large single or multiple celled structure, generally made of concrete, that uses its in-water dead weight to resist static mooring loads and suction between the soil and the structure to resist dynamic loads.

[0039] Suction piles use in-water dead weight as well as skin friction to resist loads. Suction piles are used primarily in mooring systems of permanently moored structures. However, they are also being proposed more and more for deepwater temporary MODUs (mobile offshore drilling units), particularly polyester mooring systems. Suction piles basically resemble a large can having an open bottom. The aspect ratios, length divided by diameter, typically range from two for stiffer soils to seven for softer soils. Suction piles also have various equipment located on the pile top which is used for embedment. The equipment on the pile top includes butterfly valves, pumpskid landing frame, bulls-eye (verticality check) and differential pressure gauge. To save on cost, the top plate can be designed to be removable and re-used for embedment of other piles. Suction piles are used in permanent systems because they can take a large vertical load, allowing taut leg mooring systems which offer increased performance at lower cost. Suction piles can also be used as a support structure between the seabed and a manifold.

[0040] Another example of a component 22 suitable for lowering with the apparatus of the subject invention is the suction follower. The suction follower is used for the embedment of suction embedded plate anchors (“SEPLA”) into the soil. It works under the same premise as the suction anchor, described above. The major difference is that the suction follower is recovered directly after embedment of the SEPLA. The suction follower has equipment on the top plate similar to the suction anchor.

[0041] The installation vessel 10 of the subject invention is capable of launching, lowering and installing all the above described components 22 in varied dimensions and weight by using an A-frame 12, a spreader beam 14 and motion compensator 16. The installation vessel may also utilize a remote operation vehicle (“ROV”) 20 to assist the positioning work during the installation. The vessel 10 is also preferably equipped with anchor handling/towing winches 18 and wire, rope and/or chains suitable for lowering the components that are well known to those skilled in the art.

[0042] The preferred installation vessel 10 is an Anchor Handling Tug Supply (AHTS) vessel. FIGS. 1 through 4 depict two different installation vessels properly equipped for use in the subject invention. FIG. 5 depicts an A-frame in its various positions of operation. FIGS. 6 through 9 depict a motion compensator. FIG. 10 shows a typical flow diagram for the hydraulic oil and nitrogen gas, as well as controls and valves, for a motion compensator.

[0043] Example 1 provides example characteristics of an AHTS vessel suitable for use in connection with the present invention. However, the vessel 10 of the subject invention is not limited to these characteristics provided only as by way of example.

EXAMPLE 1

[0044] The vessel 10 is an AHTS of at least 90 meters long, 23,480 BHP of main engine power and continuous bollard pull of 250 tons and having the following characteristics:

[0045] LOA=90.30 meters, Beam=23.00 meters, Depth=9.50 meters;

[0046] 17280 kW (23480 BHP) main propulsion engines;

[0047] 2×880 kW stern thrusters, 1×1500 kW bow thruster and 1×1300 kW retractable azimuth type bow thruster;

[0048] DP (AA) under Lloyd's Class (DP Class II);

[0049] Triple drum anchor handling & towing winch, maximum pull capacity 400 tonnes and 625 tons depending on drum.

[0050] 2 chain handling systems and 4×250 cubic meter chain lockers;

[0051] 2 sets of 700 tons shark jaws with towing/guide pins;

[0052] Two 4.0 meters diameter stern rollers, rated at 800 tons each;

[0053] 60 person accommodations; and

[0054] Two complete 100 hp ROV systems, one of which will be the back-up system. The ROV systems are mounted on separate platforms above the vessel's main deck.

[0055] The capacity of the vessel 10 for launching and lowering components 22 to the seafloor is limited by capacity of the A-frame 12 and the motion compensator 16, and the winching capacity of the vessel 10. For example, the maximum dimensions of the component 22 to be lifted are typically limited by the A-frame 12 size. The maximum component 22 weight is limited by the capacity of the motion compensator 16, the A-frame 12 and the winch 18.

[0056] The A-frame 12 of the subject invention allows lifting of the component 22 from the back deck 58 of the vessel 10, transferring it beyond the stern, and lowering the component 22 in the water, or vice versa, if required. As shown in FIGS. 11 and 12, the A-frame 12 may also remove the component 22 from a barge or other surface external to the vessel. Depending the installation, the A-frame 12 also provides for connections between spiral strand wire 24 and chain 26 to be lifted over the roller during deployment and while under load to prevent damage to the spiral strand spelter sockets and flex-relieve boots. In the preferred embodiment, the A-frame 12 is equipped with additional sheaves 28 in the cross member for the spreader beam 14.

[0057] As shown in FIG. 5, the A-frame 12 is generally defined by its portal height and height below sheave 28, its width at deck and top, the pivot time in an outwardly and inwardly direction, total weight, and static and dynamic moment. The capacity of the A-frame 12 must be differentiated between internal handling (on deck) and outboard handling (in water, beyond 3 meters off the pivot point) as different design factors are applicable. Furthermore, the lifting capacities vary depending on the reach of the A-frame 12. As known to those skilled in the art, the design of the A-frame 12 starts with the selection of a maximum safe working load for the structural design. It is important to determine the maximum dimensions of the component that will be traveling through the A-frame 12 as determinative of the overall size of the A-frame 12. Loads and load-factors for different sea-states such as wave height, transverse windload and total tranverse load must be taken into consideration.

[0058] The A-frame 12 is preferably pivoted by marine hydraulic cylinders 30 arranged in parallel. Deck brackets may be supplied as separate units, starboard and port side, and welded to the deck 58. The hydraulic cylinders 30 are preferably fully retracted when in stored position, protecting the piston rods from corroding and suffering unnecessary mechanical stress. Different mechanisms well known to those skilled in the art may be used in lieu of hydraulic cylinders 30 including a traveling gantry.

[0059] As an option, the control and operation of the A-frame 12 (inward and outward movement) is maintained via a remote control unit. Also, position and load monitoring control unit may be provided. To access the A-frame 12, ladders and platforms are necessary to the power pack, wire sheave 28 and certain bracket positions. Access to the A-frame 12 is based on its parked position.

[0060] For launching, lowering and installation of the components 22 on the seafloor 32, the subject invention provides load lowering with accurate control of the orientation of the component 22, and allows for adequate motion compensation for operations under swell conditions. The motion compensator 16 is defined by its wire rope stroke and the maximum diameter of its hoisting wire tracks, with identical motion compensation for up to two wires. Motion compensators operate under principles that are well known to the skilled artisan.

[0061] A motion compensator 16 corrects and compensates for varying loads created by the motions of the vessel 10 caused by wind and waves. The compensator 16 typically has hydraulic cylinders 36 or an equivalent device that will move within designed minimum and maximum amplitudes. The motion compensator 16 works to keep tension in the wire 34 constant and at a specific tension during lowering operations. The specified tension changes during lowering since the load becomes heavier as the wire 34 is payed out.

[0062] The motion compensator 16 is used to minimize the relative motion between the component 22 and the seafloor 32 as the component 22 is lowered. Motions are mainly created by waves that pass by the vessel 10 and thus cause the vessel 10 to move. This motion is usually in the form of heave. The heave at the stern of a vessel 10 is transferred to the wire 34 and thus the component 22 being supported by the wire 34 moves up and down accordingly.

[0063] As shown in FIG. 9, a preferred embodiment of the motion compensator 16 includes a 500-ton hydraulic jack 36, several sheaves 42 a, 42 b, 44 a, 44 b and guides, nitrogen and oil accumulator 38 and several nitrogen reservoirs/pumps 40. The hydraulic jack 36 is attached to a dynamic sheave 42 in the horizontal plane. The rod on the hydraulic jack 36 moves back and forth to compensate for vessel 10 motions. Since the dynamic sheave 42 is attached the hydraulic jack 36, it moves with the hydraulic jack 36. However, other types of motion or heave compensators may be suitable for use with the apparatus and method of the subject invention.

[0064] As shown with particularly in FIGS. 1, 2, and 9, a wire 34 travels from a winch 18 over the horizontal dynamic sheave 42 a, thus moving with the motion of the vessel 10. The wire 34 wraps 180° around the dynamic sheave 42 a in an opposition direction where it further wraps over a second sheave 44 a, the static sheave, on the motion compensator 16 in the vertical plane. The wire 34 wraps around the static sheave 44 a from approximately 90° to 180° depending on the arrangement. From here, as shown in FIG. 2, the wire 34 then wraps over the first sheave 28 of the A-frame 12, down to the spreader beam 14 through the spreader beam sheaves 46 and back up to the vessel and over the second sheave 28 of the A-frame 12.

[0065] The wire 34 travels further down to the deck 58 as shown in FIG. 1 and wraps around one or more deck sheaves 50 and back to the motion compensator 16. The wire 34 is then reeved a second time through a second static sheave 44 b and second dynamic sheave 42 b on the motion compensator 16. The second dynamic sheave 42 b is attached in-line to the first dynamic sheave 42 a and thus the hydraulic jack 36. Therefore, the motion of the first dynamic sheave 42 a will be identical to the motion of the second dynamic sheave 42 b. The second static sheave 44 b is next to the first static sheave 44 a in the same orientation. The component 22 is then connected to the spreader beam 14 with miscellaneous rigging 48 including, but not limited to steel wire, synthetic rope, cable, chain and the like.

[0066] Although the wire 34 may be secured to the deck 58, it is preferable to have a second dynamic sheave 42 a because it allows for the full range of use of the motion compensator 16. If only one end of the wire 34 is reeved through the motion compensator 16 as shown in FIGS. 2, 3 and 4, and thus the other end is attached to the deck 58, only half of the full range of motion of the compensator 16 can be attained. However, as shown in FIGS. 1, 9, 11-13, if the single wire 34 runs through the motion compensator 16 twice, the full amplitude of the motion compensator 16 will be achieved as a result of the way the wire 34 is reeved through the A-frame 12 and the spreader beam 14.

[0067] Furthermore, the wire 34 is double parted meaning the same wire 34 is used in two locations to lower the same load. Hence, as shown in the Figures, the wire 34 is located on both the starboard side 60 of the vessel 10 and the port side 62 of the vessel 10. Since the wire 34 is double parted, both sides of the wire 34 move with the motion of the vessel 10. If only a portion of the wire 34 moves with the vessel 10 (as when attached to deck 58) the component lowered will only move at half of the full amplitude of the motion compensator 16. The maximum sea state in which work can be performed is thus limited.

[0068] The motion compensator 16 is mounted between the winch 18 and the load (A-frame 12), limiting the wire tension between upper and lower limits. When the vessel 10 heaves for example, the load on the wire 34 naturally increases due to the friction and added mass effects of the water on the component 22 being lowered. The motion compensator 16 maintains a near constant tension and in effect keeps the component 22 relatively stationary, with respect to the seabed 32. The operation of the motion compensator is analogous to the workings of a spring where pushing on it and the spring will resist because it is designed to keep a near constant tension.

[0069] The motion compensator 16 is semi-passive, meaning that it always tries to stay in equilibrium. For example, in a preferred embodiment shown in FIG. 10, the accumulator 38 of the motion compensator 16 has two compartments that are separated by a bladder 52. On one side of the bladder 52 is hydraulic oil 54 flowing through the hydraulic jack 36. On the other side of the accumulator 38 is nitrogen 56 supplied from the nitrogen reservoirs and pumps 40. The motion of the vessel 10 causes the load on the wire 34 to increase or decrease the amount of hydraulic oil, and compressible nitrogen will contract or expand. Depending on weather the vessel 10 is heaving upwards or downwards. The dynamic sheave 42 attached to the hydraulic jack 36 is initially set at the mid-stroke of the system, which means it has an equal displacement in each direction as it is set in a mean position.

[0070] However, the motion compensator 16 is not completely passive, as the load will increase as more wire 34 is paid out. When the component 22 is first over boarded, the weight of the wire 34 is relatively small, and thus is negligible. As the component 22 is lowered the weight being suspended from the A-frame 12, and thus the load on the motion compensator 16, increases as more wire 34 hangs from the A-frame 12. This is especially true in deeper water applications, wherein the wire 34 can actually weigh as much or more than the component 22 being lowered.

[0071] The problem of additional load of the wire 34 is remedied by increasing the nitrogen pressure as more load is added due to extra weight of the wire 34. The nitrogen pressure 56 is increased in such a way as to keep the mean position of the dynamic sheaves 42 at their initial position, mid-stroke of the hydraulic jack 36. If, for example, the vessel 10 is heaving upwards, the load on the wire 34 will increase. As a result the motion compensator 16 will move the hydraulic jack 36 out to compensate for this increased load. It is effectively like instantly paying out more wire. The system works well because of the nitrogen 56 in the accumulator 38. Nitrogen 56 naturally compresses or expands because it is a gas. The pressure of the nitrogen 56 in the accumulator 38 can be calculated for a given weight, or load, on the wire. The calculation is similar to Example 2, shown below.

EXAMPLE 2

[0072] In order to theoretically calculate nitrogen pressure before starting operation when motion compensator is filly retracted, the below procedure can be followed:

[0073] 1. Determine the weight of load into water.

[0074] 2. Draw the isothermic curve from the central point

[0075] X=2 m

[0076] Y=above load

[0077]  Calculate the corresponding POVO at X=0 m (retracted position)

P ₀ V ₀ =P ₁ V ₁ ×P ₂ V ₂

[0078] When

[0079] V₀=Nitrogen Volume when fully retracted

[0080] =3×353+640=1700 liters

[0081] V₁=Nitrogen Volume when mi-extended

[0082] =3×353+320=1380 liters

[0083] P₁=Pressure at mid-extended

[0084] =Load/1.583

[0085] V₂=Nitrogen Volume when fully extended

[0086] =3×353=1060 liters

[0087] P₀ is the Nitrogen pressure when fully retracted.

[0088] 3. Draw the adiabatic curves using the above procedure but with a polytropic exponent n=1.4

P ₀ V ₀ ^(1.4) =P ₁ V ₁ ^(1.4) =P.V. ^(1.4)

[0089] Nitrogen Pressure Selection In isotherm PV = Constant P₁V₁ = P₂V₂ In dynamic adiabatic P₀V₀ ^(n) = P₁V₁ ^(n)

[0090] As shown in FIGS. 1 though 4, the subject invention also comprises a spreader beam 14 having at least a two-part lowering wire 34 providing the ability to lower and lift without twisting the wires 34 in the water. The spreader beam 14 allows for accurate heading control of the subsea structures during placement and prevents lowering wire 34 from twisting as the component 22 is lowered. The spreader beam 14 is lowered and raised from the A-frame 12 on the vessel 10, using the A-frame's 12 port and starboard sheaves 28, which are maintained a calculated distance apart. The spreader beam 14 is fitted with at least two (2) wire rope sheaves 46.

[0091] The spreader beam 14 may be a simple load bearing bar or a hollow pressure vessel. For a hollow pressure vessel, the spreader beam 14 can be made to be negatively buoyant, neutrally buoyant or positively buoyant. A neutrally buoyant or positively buoyant spreader beam 14 may have advantages over a negatively buoyant beam as it will float above the component 22 and allow an ROV 20 to work unhindered during component 22 installation operations.

[0092] The spreader beam 14 provides the apparatus of the subject invention with an inherent anti-spinning capability. For example, as torque is applied to the spreader beam 14 in a clockwise direction, the tension in the wire 34 leading to the A-frame 12 creates a rotation in the opposition direction or counterclockwise. As the load is lowered, the tension in the wire 34 will increase due to weight of the wire 34. The spreader beam 14 will equilibrate these counteracting forces until it rotates beyond 90 degrees with respect to the A-frame 12. If the spreader beam 14 is position beyond 90 degrees relative to the A-frame 12, the righting moment decreases in the face of increasing rotating forces and the wires will wind up.

[0093] The length of the spreader beam 14 is proportionally related to the maximum depth that components 22 may be lowered before the lowering wires will twist. Mathematically, the spreader beam 14 approximates a crane block. An example calculation of theoretical distance between the A-frame 12 and the spreader beam 14 at which the wires 34 will twist is provided in Example 3 below. Depending on the wire 34, the depth that a component 22 will wind up may be calculated using the following wire constants and equations provided in Example 3 below.

EXAMPLE 3

[0094] Torque values generally quoted are maximum values and are given at a load equal to 20% of the breaking strength (FOS=5). Some ropes will have non-linear torque to load curves. It is generally assumed that the rope is torque-free when unloaded. Torque is maximum on a new rope. Wear and usage diminishes the torque value, improving stability. These calculations may be conservative in other ways.

[0095] For example, if the increase in load is due to the weight of the wire, the incremental increase in rotation should be half the value that would be caused by increasing the component's 22 weight by the same amount.

[0096] Weight of manifold: W:=927·kip

[0097] Should include wire and spreader beam too. Term cancels out later in analysis.

[0098] Nominal rope diameter: d:=3·in

[0099] Number of parts: n:=2

[0100] Torque generated per part of sheave line: $\begin{matrix} {T_{part}:={T_{{fac}\quad 1} \cdot \frac{W}{n} \cdot d}} \\ {T_{part} = {8111.25\quad {{lb} \cdot {ft}}}} \end{matrix}$

[0101] Total torque generate by two-parts, an upper block or T_(tot):=n·T_(part)

[0102] the A-frame, and the lower block or the T_(tot)=16.22·kip·ft

[0103] spreader beam at 90 degree.

[0104] Solution based on graphical techniques (see diagram below):

[0105] Half A-frame (upper block) wire spacing $\begin{matrix} {a:=\frac{18.75 \cdot {ft}}{2}} \\ {a = {112.5 \cdot {in}}} \end{matrix}$

[0106] Half spreader beam (lower block) wire spacing $\begin{matrix} {b:=\frac{18.75 \cdot {ft}}{2}} \\ {b = {112.5 \cdot {in}}} \end{matrix}$

[0107] Horizontal projection of wire tension using graphical technique c:={square root}{square root over (a²+b²)}

[0108] c=159.1·in

[0109] Moment arm $\begin{matrix} {x:=\sqrt{a^{2} - \left( \frac{c}{2} \right)^{2}}} \\ {x = {79.55 \cdot {in}}} \end{matrix}$

[0110] Counter moment: $M_{counter}:={2 \cdot \left\lbrack {\left( \frac{W}{2} \right) \cdot \frac{c}{L}} \right\rbrack \cdot x}$

[0111] Righting or counter moment based on 2-part block with equal sharing of total load. Two times the horizontal projection of the wire tension times the moment arm.

[0112] L=distance between upper and lower block.

[0113] For equilibrium at 90 deg position $\begin{matrix} {M_{counter} = \quad T_{tot}} \\ {{or}{\quad \quad}} \\ {{2 \cdot \left\lbrack {\left( \frac{W}{2} \right) \cdot \frac{c}{L}} \right\rbrack \cdot x} = \quad {2 \cdot \left( {T_{{fac}\quad 1} \cdot \frac{W}{2} \cdot d} \right)}} \\ {{{then}\quad {by}\quad {reducing}}\quad} \\ {L:=\quad \frac{c \cdot x}{T_{{fac}\quad 1} \cdot d}} \\ {L = \quad {5022.32\quad {ft}}} \end{matrix}$

[0114] At this spacing between the A-frame sheaves (upper block) and the spreader beam sheaves (lower block), the angle between the A-frame and the spreader beam will exceed 90 degrees and the sheave wires will wind up only if the vertical distance between the A-frame and spreader beam exceeds L, which essentially represents the water depth. Units: Constants: pcf: = lb · ft⁻³ Constants provided by AMCLYDE in Deepwater Lowering ton: = 2000 · lb and Handling Hardware for the Offshore Industry kip: = 1000 · lb plf: = lb · ft⁻¹ psf: = lb · ft⁻² Torque factor for 6 × 19, 6 × 36, 6 × 41 and T_(fac1): = 0.07 psi: = lb · in⁻² DyForm 6 wires ksi: = 1000 · lb · in⁻² Torque factor for 18 × 7 wire T_(fac2): = 0.066 Torque factor for Paragon wire T_(fac3): = 0.043 Torque factor for DyForm 18 wire T_(fac4): = 0.058 DyForm 35LR wire T_(fac5): = 0.019

[0115] As shown in the figures, the installation vessel 10 is generally set up having a wire 34 or rope 34 wound tightly on a winch and secured. The outboard end of the wire 34 is drawn or reeved through the motion compensator 16. The wire 34 is further reeved through a sheave 28 on one side of the A-frame 12. The wire 34 is further reeved through the spreader beam 14. The wire 34 is then reeved back through the A-frame 12. The wire 34 may be reeved through the motion compensator 16 and then attached and secured to the deck 58 of the AHTS. Alternatively, the wire 34 is attached and secured to the deck 58 of the AHTS vessel 10.

[0116] More particularly, the wire 34 or rope 34 is spooled onto the winch 18 having one end secured into the winch 18. This end is referred to as the inboard end or deadmanned end. The other end of the wire 34, the outboard end, is then reeved through the motion compensator 16. The wire 34 then leads to and partially around one of the two or more sheaves 28 on the A-frame 12. The sheaves 28 are positioned on the A-frame 12 so that wire 34 can travel safely over the A-frame 12 and not damages either the A-frame 12 or the wire 34. The sheaves 28 run in a direction parallel too the installation vessel 10 and motion compensator 16.

[0117] The A-frame 12 provides a separation or distance between the wire 34 and makes the system act similar to a crane block. Directional control of the component 22 is thus attained, and this is particularly important before the component 22 is set down on the seafloor 32. Furthermore, without directional control, the wire 34 may suffer damage during installation when the wire 34 unwinds.

[0118] Once the wire 34 is over the first sheave 28 on the A-frame 12, the wire 34 travels down to the first sheave 46 a on the spreader beam 14. The spreader beam 14 may be a long tube having two or more sheaves 46 running perpendicularly to the installation vessel's 10 heading and to the sheaves 28 on the A-frame 12. The wire 34 further travels along the length of the spreader beam 14 and runs over a second sheave 46 b. From the second sheave 46 on the spreader beam 14, the wire 34 runs backup towards the installation vessel 10 and over a second sheave 28 b on the A-frame 12.

[0119] Once over the second sheave 28 b of the A-frame 12, the wire 34 is drawn back down to the deck 58 of the vessel 10 and is then either: (a) deadmanned (secured) to the deck 58; (b) further reeved through the motion compensator 16 and secured to the deck 58, or (c) reeved through the motion compensator 16 and spooled up onto a second winch 18. The wire 34 may be secured to the deck 58 by a steel padeye. The advantage of running the wire 34 through the motion compensator 16 again is the that range of motion or the maximum seastate is essentially doubled as both sides of the wire 34 will move as opposed to only one side moving. The advantage of running the wire 34 through the motion compensator 16 and then a second independent winch is that the speed of lowering the component 22 may be increased since two winches will be lowering as opposed to one.

[0120] In yet another embodiment of the invention, a lowering wire 34 rigging arrangement includes a lowering wire 34 stored on the portside half of the AHTS upper winch drum 18. The long upper drum will be provided with a separation flange 64 for this purpose. A level wind (not shown) is provided. The wire 34 will pass through the level wind, and through a fairlead 50 mounted on the forward end of the motion compensator 16. From the fairlead 50, the wire 34 passes around one of the compensator's internal load sheaves 42 and goes forward over a fixed sheave 44 on the forward end of the motion compensator 16. From this fixed sheave, the wire 34 runs over the portside A-frame sheave 28 a, and down to the spreader beam 14. Next, the wire 34 passes through the portside spreader beam sheave 46 a and runs along the top of the beam to the starboard spreader beam sheave 46 b. After passing through the starboard spreader beam sheave 46 b, the wire 34 will go up and over the starboard A-frame sheave 28 b, through a couple of deck mounted fairlead sheaves 50, and through the second compensator sheave 42 b. The end of the wire 34 is attached to a deadman padeye 66 on the deck 58, or a deadman anchor that will be integrated into the motion compensator 16. Rather than connecting the end of the lowering wire 34 to a dead-man anchor 66 as discussed above, as an alternative, this wire 34 may lead through fair leading sheaves 58 and through a level wind to the starboard half of the upper drum 18. This will allow faster lowering and recovery operations, and result in fewer layers on the winch drum 18.

[0121] Typically, components 22 are delivered to the vessel's 10 maindeck 58 on skid frames. The components 22 to be installed are skidded underneath the retracted A-frame 12. Once the structure has been lifted off the skid frame, the A-frame 12 is extended fully aft, and the component 22 is lowered in the water.

[0122] As mentioned above, to lower a component into the sea using the apparatus of the subject invention, the installation vessel 10 connects the spreader beam 14 to the component 22. The component is attached to the spreader beam 14 with miscellaneous rigging 48. The suction pile 22 is released from the barge 68 it is sitting atop. The vessel 10 moves forward and pulls the component 22 off of the barge 68.

[0123] As shown in FIGS. 11 through 14, the component 22 (shown as a suction pile in the figures) swings down into the water until it is in a static position. The motion compensator 16 dampens the motion. The wire 34 on the winch 18 is paid out, as well as the wire 24 and chain 26 until the component 22 arrives above the seabed 32. The ROV 20 inspects the component 22 and the seabed 32 prior to set down to confirm that the relative motion between the component 22 and the seabed 32 is minimal. The winch 18 pays out more wire 34 until the component 22 embeds into the seabed 32. A third of the component 22 is embedded as a result of the component weight.

[0124] The ROV 20 docks the component 22 and pumps it into the soil. The ROV 20 then proceeds to detach the miscellaneous rigging 48 from the spreader beam 14. The spreader beam 14 is now recovered to the installation vessel 10 by heaving in on the winch 18 and thus picking up the installation wire 34. The vessel then proceeds to install the rest of the mooring wire 24 and chain 26.

[0125] The subject invention is suitable for use with numerous methods of installing subsea components 22. For example, the apparatus of the subject invention is useful in connection with the method of installing the plate anchor and suction follower disclosed in U.S. patent application Ser. No. 09/627,873 incorporated herein by reference in its entirety.

[0126] While this invention has been described with a reference to illustrative embodiments, this description is not intended to be construed in a limiting sense. Various modifications and combinations of the illustrative embodiments as well as other embodiments of the invention, will be apparent to persons skilled in the art upon reference to the description. It is, therefore, intended that the appended claims encompass any such modifications or embodiments.

[0127] Although the preferred embodiments of the invention have been illustrated in the accompanying Drawings and described in the foregoing Detailed Description, it will also be understood that the invention is not limited to the embodiments disclosed, but is capable of numerous rearrangements, modifications and substitutions without departing from the spirit of the invention. 

What is claimed is:
 1. An apparatus for installation of subsea component comprising: a vessel having a motion compensator and an A-frame wherein said motion compensator is connected to said A-frame; and a spreader beam wherein the component is connected to the spreader beam and said spreader beam is moveably connected to said A-frame for lowering the component into the sea.
 2. A method of installing a component onto the sea floor into comprising the steps of: a) connecting a spreader beam to an A-frame positioned on a vessel; b) linking the component to a spreader beam; c) lowering the component into the sea; d) controlling varying load on the component and lowering wire by use of a motion compensator operatively connected to the A-frame; and e) positioning the component on the sea floor.
 3. In a method for installing a subsea component onto the seafloor using a vessel having an A-frame the improvement comprising: Controlling component orientation of a subsea component as it is lowered into the sea with a motion compensator, said motion compensator counterbalancing and offsetting variations in load on the component created by the movement of the vessel caused by wind and waves or heave. 